a) Field of the Invention
The present invention is directed to a method and an arrangement for determining the thickness, distance and/or profile of areas of a transparent and/or diffuse object that are spaced apart. In particular, the solution is suitable for measuring partial distances, i.e., distances between surfaces, interfaces or defects in the eye. The measurement of these partial distances is especially important for cataract surgery and refractive eye surgery.
b) Description of the Related Art
The length of the individual axial eye portions can be determined by means of acoustical or optical length measurement methods. Short-coherence interferometry has been applied to an increasing extent for this purpose owing to the advantages of its contactless, highly precise manner of operation.
For short-coherence interferometry, arrangements based on the Michelson principle are generally used, wherein the beam of a short-coherent radiation source is split into a measurement beam and a reference beam. When the coherence length of the utilized light is less that the optical path length between the interfaces to be measured, no interference occurs between the light bundles reflected by the interfaces. When the measurement beam and reference beam are recombined, the change in path length effected in the reference beam path by means of a reference mirror leads to interference in case the path lengths of the measurement beam and reference beam are identical. The change in path length can be effected, for example, by a translational movement of the reference mirror (DE 32 01 801 C2) or by rotation of a transparent cube (WO 96/35100). The occurring interference patterns are directed to a detector and evaluated in a corresponding manner. The change in path length of the reference beam is a direct measure of the queried distance between the interfaces of the eye.
In classic short-coherence interferometry, the reference mirror travels a path length corresponding to the distance to be measured, while the measured object is at rest. Since it is difficult to fixate an eye for the period of time required to measure distances of approximately 30 mm, special solutions were developed for ophthalmologic applications which also enable measurements of living objects. Measurement errors caused by inadequate fixation of the eye to be measured can be avoided through scanning lengths of only a few millimeters.
In the dual beam method, as it is called, areas of an eye that are spaced apart with respect to depth are illuminated/scanned by two measurement beams simultaneously. The solution described in DE 32 01 801 C2 uses measurement beams of different wavelength which are focused on the cornea and, for example, on the fundus by means of a diffractive optical element. The interferometer arrangement is adjusted to the distance to be measured, for example, between the cornea and fundus, so that a scanning length of only a few millimeters is required.
WO 01/38820 A1 describes a solution in which two areas of an eye which are spaced apart with respect to depth are illuminated/scanned by means of a dual beam. A partial beam is cut out of the measurement beam focused on a first interface in front of the measured object, directed by a so-called diverting unit, and focused on a second interface of the eye. Accordingly, a single measurement encompasses reflections at a plurality of interfaces of the eye almost simultaneously. The beams have different optical characteristics such as, e.g., wavelength, polarization state, or the like, so that the individual reflections can be distinguished from one another. The evaluation of the two measurement beams is carried out through path length change in the reference beam; different interference patterns are also generated for the different measurement beams.
However, the described arrangements have the disadvantage that the measurement beams illuminate/scan two or more interfaces simultaneously so that the radiation not contributing to the measurement generates background interference and noise. The less precise the adjustment of the interferometer arrangement to the distance to be measured, the greater its required scanning area.